An optical modulator is a device for modulating an optical signal, such as an optical carrier wave, by an electrical signal. Some property of the optical carrier wave, such as phase, amplitude, polarization, etc., may be modulated by the electrical signal by using an electro-optical effect of some kind, such as Pockels effect, electro-absorption effect, photoelasticity, etc. The electrical signal may need to be amplified to provide a degree of modulation of the optical carrier wave adequate for a particular application. Linear amplifiers are commonly used to provide amplification of an electrical signal for optical modulation, because linear amplifiers preserve the waveform of the electrical signal being amplified.
Referring to FIG. 1, a conventional modulator apparatus 100 includes a linear amplifier 102 connected to an optical modulator 104. In operation, a high-frequency electrical signal 106 creates a running electromagnetic wave 108, which propagates in a RF waveguide formed by electrodes 110 running parallel to an electro-optical waveguide 112. The electromagnetic wave 108 runs toward an RF termination 114. The electromagnetic wave 108 propagates in the electro-optical waveguide 112 at the same speed as light 114. As a result, the light 114 becomes effectively modulated.
The linear amplifier 102 provides a constant gain, which is independent on the input signal: Vout=GVin, where Vin is the input voltage, Vout is the output voltage, and G is a constant. Referring to FIG. 2, the output voltage of the linear amplifier 102 is plotted against the input voltage. In this example, the input voltage range is −0.5V to 0.5V, and the corresponding output voltage range is −1V to 1V, so that the gain G=2.
Referring back to FIG. 1, the linear amplifier 102 is typically constructed to provide a significant amount of RF power for generation of the running electromagnetic wave 108. To drive the electrodes 110 in a linear fashion at high speed, a typical linear driver amplifier may consume twenty times more power than electrical power delivered to the optical modulator 104. This factor of power consumption is referred to as “overhead.” Often, conventional linear modulators have a significant overhead.
Furthermore, to provide linear output, the electrical signal swing may need to be wider than typical output modulation range, to ensure that the output will be sufficiently close to linear in an operational range of interest. The requirement of linearity may sacrifice other performance metrics, such as amplifier power consumption or operating speed.